Since a polyolefin such as polypropyrene, high-density polyethylene, straight chain low-density polyethylene or the like is excellent in mechanical properties, chemical resistance and the like and is very useful in respect of a balance of economy, it has been widely utilized in every molding field. However, the polyolefin has small melt tension and a low crystallization temperature. For this reason, molding properties such as hollow molding, foam molding, extrusion molding and the like are poor and the high-speed productivity of a mold has limitations in various molding methods.
A method for causing polypropylene to react with organic peroxide and a crosslinking auxiliary agent in the melting state (Japanese Unexamined Patent Publication Nos. 59-93711, 61-152754 and the like), a method in which a low decomposition temperature peroxide is caused to react with semicrystalline polypropylene in the absence of oxygen to produce polypropylene which has free end long chain branch and does not contain gel (Japanese Unexamined Patent Publication No. 2-298536) and the like have been disclosed as a method for increasing the melt tension and crystallization temperature of polypropylene.
A composition in which polyethylene having different intrinsic viscosity or molecular weight or polypropylene are blended and a method for producing the same composition by multistep polymerization have been proposed as another method for enhancing melt viscoelasticity such as melt tension or the like.
There have been disclosed a method for adding 2 to 30 weight parts of superhigh molecular weight polypropylene to 100 weight parts of ordinary polypropylene and extruding a product at a temperature which is equal to or higher than a melting point and equal to or lower than 210.degree. C. (Japanese Examined Patent Publication No. 61-28694), a method of preparing an extrusion sheet made of polypropylene which is obtained by a multistep polymerizing method and contains 2 components having the limiting viscosity ratio of 2 or more and different molecular weights (Japanese Examined Patent Publication No. 1-12770), a method for producing a polyethylene composition which contains 1 to 10 weight % of polyethylene having high viscosity-average molecular weight and comprises three kinds of polyethylene having different viscosity-average molecular weights by a melting and kneading method or a multistep polymerizing method (Japanese Examined Patent Publication No. 62-61057), a method for polymerizing superhigh molecular weight polyethylene having a limiting viscosity of 20 dl/g or more with 0.05 to less than 1 weight % according to a multistep polymerizing method using an active titanium-vanadium solid catalyst component (Japanese Examined Patent Publication No. 5-79683), a method for polymerizing 0.1 to 5 weight % of superhigh molecular weight polyethylene having a limiting viscosity of 15 dl/g or more by using an active titanium catalyst component which has preliminarily been polymerized with 1-butene or 4-methyl-1-pentene according to a multistep polymerizing method using a polymerization container having a special arrangement (Japanese Examined Patent Publication No. 7-8890) and the like.
Furthermore, there have been disclosed a method for producing polypropylene having high melt tension in which propylene is polymerized with a support type titanium-containing solid catalyst component and an organic aluminum compound catalyst component by using a preliminary polymerization catalyst which is prepared by preliminarily polymerizing ethylene and a polyene compound (Japanese Unexamined Patent Publication No. 5-222122), and a method for producing an ethylene-.alpha.-olefin copolymer having high melt tension by using an ethylene containing a preliminary polymerization catalyst which contains polyethylene having a limiting viscosity of 20 dl/g or more that is obtained by preliminarily polymerizing only ethylene using the same catalyst component (Japanese Unexamined Patent Publication No. 4-55410).
According to various compositions and producing methods which have been proposed in the prior art, the melt tension can be enhanced to some extent but the residual odor of a crosslinking auxiliary agent, crystallization temperature, thermal stability and the like should be improved.
The process for manufacturing high molecular weight polyolefin should be modified for the following reasons. More specifically, it is hard to precisely control the amount of olefin (co-)polymerization in order to generate a small amount of polyolefin having a high molecular weight in the multistep polymerizing method which is to be incorporated into the ordinary olefin (co-)polymerizing step for polymerization. In addition, the polymerization temperature should be lowered to generate polyolefin having a molecular weight which is sufficiently great. Furthermore, the productivity of the final polyolefin composition is lowered.
In the method for preliminarily polymerizing a polyene compound, it is necessary to prepare a polyene compound separately. In the method for preliminarily polymerizing polyethylene, the dispersibility of the preliminarily polymerized polyethylene to the polyolefin which is finally obtained is non-uniform. Consequently, further improvement should be required in respect of the stability of the polyolefin composition.
According to the prior art, the melt tension and the crystallization temperature of polyolefin are insufficiently enhanced as described above. In addition, there are problems to be solved with respect to odor and thermal stability. Furthermore, it is necessary to enhance the productivity of such polyolefin.